Environmental context. Humic and fulvic acids are macromolecular, multifunctional, polyacidic compounds that are important proxies for humic-like substances (HULIS), which are ubiquitous components of tropospheric particulate matter. The hygroscopic nature of these substances suggests that they can contribute to direct and indirect climate forcing. Thus, the effects of water uptake in humic-like particles in the atmosphere must be well understood.

Abstract. The water uptake of humic and fulvic acid aerosols was determined by hygroscopic tandem differential mobility analysis (hTDMA) and extinction Fourier transform infrared (FTIR) spectroscopy. Water uptake on humic and fulvic acid thin films was also investigated using attenuated total reflectance (ATR) FTIR spectroscopy. The hygroscopic growth of monodisperse, 100-nm (dry) Suwannee River fulvic acid (SRFA) and humic acid sodium salt (NaHA) aerosols was determined and modelled using Köhler theory. A single parameter, the ionic density (ρ_ion), which contains physical properties that are not well established for these substances, was determined for SRFA and NaHA to be 2.1 × 10^–3 and 7.0 × 10^–3 mol cm^–3 respectively. The hygroscopic growth was then modelled using the ρ_ion-Köhler equation and the critical parameters determined. The critical percent supersaturation of SRFA and NaHA was determined to be 0.60 and 0.33% respectively using the surface tension of water; and 0.35 and 0.19% respectively using the surface tension of aqueous HULIS. κ-Köhler theory, was also used to calculate the critical supersaturation and was found to be in good agreement with the ρ_ion representation. Both extinction FTIR of aerosols and ATR-FTIR absorption measurements of thin films confirm enhanced water uptake with increasing relative humidity (RH).